Preparation of metal foams of aluminum

ABSTRACT

1. PROCES FOR INCREASING THE VISCOSITY OF MOLTEN SUBSTANTIALLY PURE ALUMINUM WITH A GASEOUS VISCOSITY-INCREASING AGENT, SAID PROCESS COMPRISING INTRODUCING INTO SAID ALUMINUM AT A TEMPERATURE OF FROM ABOUT 20* TO ABOUT 90* C. ABOVE THE LIQUIDUS TEMPERATURE OF SAID METAL FROM ABOUT 0.02 TO ABOUT 10 GRAMS OF AIR PER EACH 100 GRAM PORTION OF METAL.

United States Patent 3,843,353 PREPARATION OF METAL FOAMS OF ALUMINUM Leonard M. Niehylski, Birmingham, and Chester P. .larema, Detroit, Mich., and Thomas E. Lee, Baton Rouge, La., assignors to Ethyl Corporation, Richmond,

a. No Drawing. Application Mar. 10, 1971, Ser. No. 123,099,

which is a continuation-in-part of abandoned application Ser. No. 63,666, Aug. 13, 1970, which in turn is a continuation-in-part of abandoned applications Ser. No. 800,724 and Ser. No. 800,745, both Feb. 19, 1969. Divided and this application Feb. 5, 1973, Ser. No.

Int. Cl. C21b US. C]. 75-20 F 5 Claims ABSTRACT OF THE DISCLOSURE REFERENCE TO RELATED APPLICATIONS This application is a division of application Ser. No. 123,099, filed Mar. 10, 1971, which is a continuation-inpart of pending application Ser. No. 63,666, filed Aug. 13, 1970, and now abandoned which in turn is a continuation-in-part of application Ser. No. 800,724 and application Ser. No. 800,745, both filed Feb. 19, 1969, and now abandoned.

BACKGROUND OF THE INVENTION Foamed metals have been described; see, for example, US. 2,895,819; 3,300,296; and 3,297,431. In general, such foams are produced by adding a gas-evolving compound to a molten metal and heating the mixture to decompose the compound to prepare blowing gas. The gas expands causing the metal to foam. After foaming, the resultant body is cooled to form a foamed solid. The gas foaming solid can be a metal hydride such as TiH ZrH or lithium hydride, US. 2,983,597.

Many prior art foams have cells of non-uniform structure or undesirable large size. A main reason for this is the inability to evolve blowing gas and disperse it throughout the mass to be foamed at optimum rates. Thus, a need exists for a method of increasing the viscosity of the molten metal to aid in the blowing step. This invention satisfies that need.

SUMMARY OF THE INVENTION Accordingly, this invention pertains to a method for increasing the viscosity of molten metals and the viscous melts produced thereby. Metals of particular interest are aluminum and its alloys. Thus, this invention provides a process for increasing the viscosity of a molten aluminum base metal which comprises adding to said metal a viscosity-increasing amount of a viscositv-treating agent. Preferably, the aluminum base metal is an aluminum alloy; however, aluminum itself can be employed in this invention. Of the alloys, those in which a major portion is aluminum are preferred. Moreover, it is preferred that the aluminum be alloyed with a metal selected from mag- 3,843,353 Patented Oct. 22, 1974 "ice nesium, titanium, copper, Zinc, manganese, tin, and silicon.

The viscous nature of the melts produced by this process suggest use wherever it is desired to diminish fiow characteristics of molten metals; for example, where it would be advantageous to restrict the flow of molten metals through conduits in metallurgical processing. Moreover, an important utility of these melts is their surprisingly advantageous use in preparation of superior quality metal foams.

An aspect of this invention comprises the improved step of foaming a molten metal melt containing a viscosityincreasing amount of a viscosity-increasing agent. Moreover, important aspects of this invention comprise foaming a melt made more viscous by a gas or vapor; for example, carbon dioxide or steam respectively. As illustrated below, viscosity-increasing agents introduced into a molten metal mass in the practice of this invention, achieve thickening Without introduction of a solid substance derived from a non-volatile component of the thickening agent. For thickening by this invention, a decomposition reaction to produce a volatile component is not required.

Accordingly, this invention provides a method for producing an aluminum base foam which comprises (a) increasing the viscosity of a molten aluminum base metal with a viscosity-increasing amount of a viscosity-increasing agent, and (b) treating the viscous melt thereby produced. In this operation, the thickened system is heated sufiiciently to thermally decompose the blowing agent to release gas which makes the foaming take place.

Upon cooling, a solid foam is produced. Such foams produced by this invention are characterized by a surprising degree of uniformity in pore size and configuration. They can be used as structural materials especially where it is advantageous to have a light metal construction; for example, in trailer walls, doors and floors, aircraft decking, sandwich wall constructions, curtain walls, etc.

Ordinarily molten aluminum and its alloys have viscosities akin to water. When such metals are treated with a viscosity-increasing agent in accordance with this invention a much thicker melt can be produced. Generally speaking, the thickness is proportional to the amount of agent added. In fact, it is possible to make a material so thick that it is stirred with difliculty by powerful stirring devices.

Viscosity, as the term is used herein, refers to fluidity of a liquid. (In a technical sense, fluidity is the reciprocal of viscosity or apparent viscosity.) A liquid will flow slowly (have less fluidity) when the viscosity is increased. There are two types of viscosity, true viscosity and apparent viscosity. Apparent viscosity refers to the viscosity equivalence in appearance and mobility of a fluid which when measured with a viscometer evidences no or only a slight change in true viscosity. An example of a material exhibiting apparent viscosity is whipped cream. It is not known whether the viscosity increasing treatment of this invention results in an increase of true and/or apparent viscosity. Nevertheless, the above viscosity increasing agents, for example, can change an aluminum base metal from a material having about the same resistance to flow as water, to one with much less fluid. It appears that the increase in viscosity is a major increase in apparent viscosity and a minor increase in true viscosity. It has been found that treatment of an aluminum alloy having 7 percent magnesium with a viscosity increasing agent increased the viscosity (according to viscosity measurement) only about 16 centipoises. Nevertheless, when such a molten alloy is treated, according to this invention it is possible to prepare a viscous melt very resistant to pouring out of a spoon even when the spoon is turned over.

In foams produced by the process of this invention, pore size is smaller and more uniform. Moreover, the use of a viscosity increasing agent makes it possible to use less foaming agent than would otherwise be required, the reduction in amount of foaming agent being greater than that provided by any expansion of the alloy due to the presence of the viscosity increasing agent. For example, when carbon dioxide is used and ZrH is the foaming agent, 0.6 gram of ZrH will give the same expansion that 1.0 gram thereof provide in the absence of CO pretreatment. This provides a considerable saving in the cost of foaming.

One can calculate how much gas is required to achieve a desired amount of foaming. The amount of gas is conveniently expressed in theories and one theory is the amount of gas which would be generated (if the foaming agent completely decomposed) to produce a known void volume in a mass (conveniently expresed in pounds per cubic foot density or g./cc. of foam). For 15 pounds per cubic foot density, 2.5 to 3.0 theories of TiH are required and this is equivalent to 0.8 to 1.0 gram TiH per 1000 grams of metal. However, after CO treatment, to make an equivalent foam, only 1.2 to 1.7 theories or 0.4 to 0.6 gram of TIH2 are required.

DESCRIPTION OF PREFERRED EMBODIMENTS In the main, this invention pertains to the discovery that certain agents increase the viscosity of molten aluminum base metals. Another important discovery to which this invention pertains is that viscous aluminum base metals yield superior metal foams. We have also discovered that non-stoichiometric metal hydrides can be advantageously used as blowing agents. Other discoveries will be apparent from the description of this invention which follows.

There is a number of important aspects of this invention. Among these, the following are illustrative.

(1) Process for increasing the viscosity of molten aluminum base metals, and the viscous melts produced thereby;

(2) Process for preparation of improved metal foams comprising blowing viscous molten aluminum melts, and the foams produced by this process; and

(3) Use of non-stoichiometric metal hydrides as blowing agents, and foams produced by use of these agents.

With regard to metals, this invention is primarily directed to aluminum and its alloys. Commercially available aluminum can be used. Typically, such aluminum is 98.5 to 99.9 percent pure. The common impurities are silicon, tin, and magnesium, and the amount of such impurities is from 1.5 to 0.01 weight percent. Commercially available aluminum metal preparations include aluminum Alloy-3S (98 percent Al, 1.25 percent Mg), Alclad l7-ST (99.7+ percent aluminum), and Aluminum 28 (99.2 percent aluminum). Preferred aluminum alloys which can be used contain a major amount of aluminum. More preferably, such alloys contain at least 65 percent aluminum. Thus, besides aluminum, other applicable aluminum based materials contain up to 35 percent of one or more alloying elements. More preferably, such alloys contain, besides aluminum, a second metal selected from magnesium, titanium, copper, zinc, manganese, tin and silicon. With regard to these alloying elements, the following table indicates the preferred and most preferred concentrations For the purposes of this invention, substantially pure aluminum refers to aluminum having a purity of at least 98.5 (weight) percent, and aluminum base metal refers to such aluminum and aluminum alloys wherein the aluminum content is at least about 65 (weight) percent.

With regard to viscosity-increasing agents, air, oxygen, nitrogen, carbon dioxide, argon and water can be used. All of these are in the gaseous state at the temperatures of the molten metals employed. However, the physical state of the viscosity-increasing agent is not critical. Thus, for example, CO may be incorporated by adding it to the molten metal either in the solid or liquid state; water may be employed as liquid water, ice, or steam; and nitrogen may be used as a gas or as the cryogenic liquid. For the purposes of this invention, the term gaseous viscosity increasing agent can be used to describe the thickening agents provided by the invention.

FURTHER CONSIDERATIONS PERTAINING TO THICKENING To achieve thickening, the viscosity increasing agent is added to the molten metal and solidification of the metal is avoided by operating at suificiently high temperatures, usually within the range of from about 20 to about C. above the metals liquidus point. It is to be noted that the increase in viscosity is of shorter duration at higher temperatures. At the same time, if higher operating temperatures are required, more viscosity increasing agent can be employed either initially or, if desired, by incrementally mixing with the melt as it begins to thin.

In general, the operating temperatures used for thickening are over 550 C. but less than 760 C. For many aluminum base metals the temperature range may be from about 650 to about 720 C. At these temperatures the viscosity increase is fairly permanent. In many instances, the increase will last about a half hour or more. This is suflicient time for further processing, such as preparing a foam. The viscosity increase will remain even after short excursions to somewhat higher temperatures. If it is desired to hold the viscosity for longer periods, more viscosity increasing agent can be added when the molten metal starts to thin.

To achieve viscosity increase, the viscosity increasing agent must be admixed within the molten metal; it is not enough to merely treat the surface of the melt with the agent. This can be done by any mixing technique known in the art such as efiicient stirring.

Since loss of some of the viscosity increasing agent when mixing is unavoidable due to the disparity between the temperature of the molten metal and the viscosityincreasing agent this must be allowed for a determining the amount used. At a stirring rate of to about 3000 r.p.m., generally from about 0.02 or less to about 10 or more grams of thickening agent are employed per each 100 grams of metal. Good results are obtained using 0.25 to about 6 grams of thickening agent per each 100 grams of metal. A preferred general range is from about 0.1 to 5 grams of thickening agent per each 100 grams of metal, and a more preferred range is from about 0.5 to 5 grams of thickening agent per each 100 grams of metal. In any particular instance it might be beneficial to use:

even greater amounts, up to about 20 grams of thicken-- ing agent per each 100 grams of metal.

The specific amount of thickening agent used depends: to some extent upon which agent is selected. For example, with carbon dioxide, thickening is obtained using amounts. from about 0.05 gram to 8 or more grams per each 100 grams of metal. Better results are generally obtained using from about 0.1 to about 8 grams of CO per 100 grams of metal. By way of example, a thickened molten metal can be formed using from 0.1-l grams of CO per 100 grams of metal and, in fact, excellent foams have been produced using as little as 0.3-0.7 grams of CO per 100 grams of metal. Likewise, very stable thickened molten metal can be obtained using from l-8 grams of CO per each gram of molten metal.

When Water is used a preferred range is from 1-6 grams of H per each 100 grams of metal. With nitrogen the preferred range is from 1-4 grams of N per each 100 grams of metal. Likewise, with argon good results are obtained using from 14 grams of argon per each 100 grams of metal.

When using oxygen to thicken the molten metal good results can be obtained using from about 0.02 or less to about 5 or more grams per 100 grams of metal. A preferred amount is from about 0.07 to 5 grams per 100 grams of metal, although excellent results have been obtained using as little as from 0.070.5 grams per 100 grams of metal. Likewise, good thickening leading to excellent foams results from the use of from about 1-5 grams of oxygen per 100 grams of metal.

With air, a useful thickening range is from about 0.2 or less to about or more grams per gram of metal. A preferred range is from about 0.5-5 grams per 100 grams of metal. A still more preferred range is from 0.5-3 grams of air per each 100 grams of metal. Excellent results have been achieved using from 1-2 grams of air per each 100 grams of metal and, hence, this is a most preferred range.

The viscosity-increasing agent should be rapidly added, preferably in a period of seconds to three minutes. The time may be increased if a proportionately greater amount of agent is used. Generally the foaming will be effected within about one-half hour of treating with the viscosityincreasing agent.

The pressure at which the viscosity-increasing agent is added to the molten metal is not critical. In most instances, there is not much advantage in using pressures less than ambient and ambient pressures are preferred. However, in some instances, increased pressure, say up to 1500 p.s.i.g., may be used to advantage. Such pressures can tend to force the foaming agent to mix with the molten metal, the closed vessel and increased pressure retarding escape of the viscosity-increasing agent.

The viscosity-increasing agents of this invention do not act in all respects like a foaming or blowing gas such as hydrogen. They give much less expansion than an equivalent amount of hydrogen. For example, the expansion due to CO is in some instances only A; to of the expansion due to an equivalent amount of hydrogen. In some instances, the expansion from CO is less.

In many instances, the following general rules can be recited as applicable:

(1) For a given amount of viscosity-increasing agent, the viscosity increase is proportional to the amount of second metal alloyed with aluminum.

(2) The viscosity increase is proportional to the amount of viscosity-increasing agent employed.

(3) The higher the temperature, the shorter the duration of viscosity increase.

(4) The higher the temperature, the greater the amount of agent required to provide a given viscosity increase.

FOAMING A wide variety of foaming or blowing agents may be used in the process of this invention and these are generally known in the art. Of the known foaming agents, it is preferred to use metal hydrides. Of these, those which decompose to yield gaseous hydrogen at the temperature of the material to be foamed and release the gaseous hydrogen at not too fast a rate are preferred, especially titanium, hafnium or zirconium hydrides, and more especially the latter. The dihydrides of commerce as well as annealed hydrides of less than stoichiometric composition maybe used, if desired.

The amount of hydride (or other foaming agent) employed is dependent upon the amount of foaming desired, less foaming agent being required for a dense foam than for a light foam. For many uses, it is preferred to prepare foams having a density of percent or less; i.e., foams which weigh no more than about 20 percent of the weight per given volume of the unexpanded metal. For uses such as in die casting, densities of 30 percent are useful.

For such foams about 0.2 to about 1.0 gram of commercial HfH Til-I or ZrH per grams of alloy to be foamed may be used. A preferred range is from about 0.5 to about 0.6 gram.

A lower temperature limit must be exceeded in order to prepare foam, namely a temperature above which the metal to be expanded is molten and a temperature above that required to yield the gas required to do the foaming. It is also desirable to use a foaming temperature at which the blowing gas is not formed at an uncontrollable rate. Further, it is desirable to use a temperature at which the melt is sufficiently viscous. With such considerations in mind temperatures within the range of from about 550 to about 780 C. may generally be used and temperatures of from about 670 to about 705 C. are preferred.

It is preferred to carry out the foaming process at ambient pressure although greater or lesser pressures may be used if desired. In many instances, no special benefit is gained from sub-atmospheric pressures and such pressures are deleterious since they can encourage evolution of gas outside the confines of the mass to be foamed. When it is desired to use superatmospheric pressures, pressures up to 1500 p.s.i.g. or higher may be used.

When carrying out the foaming step of this invention, the foaming agent should be admixed with the mass of melt to be foamed and the more uniform the mixing the better the foam. Any known manner of mixing which provides efiicient mixing of materials in liquids can be used, but it is preferred to use efficient stirring, so that the foamng agent metal mixture is substantially homogeneous in as short a time as possible. In most instances, good results are obtained when the foaming agent-metal mixture is substantially homogeneous in a minute or less, but it is preferred to use stirring which will achieve substantial homogeneity in about 10 seconds.

To further understand the preparation of a metal foam according to the process of this invention the following preferred exemplary procedure is given:

(1) Heat about 4000 grams of alloy in an induction pot to 700 to 760 C.

(2) After melting, stir the melt so that substantially all of it moves say, with a high viscosity head at about 100 to 3000 r.p.m.

(3) Add 10 to 60 grams of powdered CO per thousand grams of alloy over a period of 1 to 3 minutes. Some flaming and acetylene-like odour may occur.

(4) Bring the viscous mixture to the temperature desired for the foaming step, 650 to 800 C.

(5) Add enough foaming agent. to give the foam the density desired. For example, when using zirconium hydride:

(a) about 1.2 g. Zr H per 100 g. of alloy for a 9 to 11 pounds per cubic foot (p.c.f.) dense product having A; to A" pores,

(b) about 0.9 g. ZrH per 100 g. alloy for an 11 to 14 p.c.f.

product to A5" pores,

(0) about 0.6 g. ZrH per 100 g. alloy for a 15 to 18 p.c.f.

product A; to pores.

The addition can be conducted while stirring, say 100 to 3000 rpm, and after all of the foaming agent is in, increasing the stirring rate to 6000 to 10,000 r.p.m.

(6) The actual foaming can be conducted in a mold whose inner surfaces define the shape of the foamed product. The mold may be open or closed as desired. When open the top surface of the product will usually be irregular in shape. When the mold is closed all surfaces of the foamed product can be as defined by the mold and usually, there is less blow-hole or large pore occurrence.

When performing a batch foaming process, there is little time to spare between the addition of the foaming agent and the production of gas, especially if the temperature of the mass to be foamed is comparatively high. This means that to disperse the foaming agent uniformly, quick dispersion must take place. This can be accomplished with high speed stirring which can optionally be used with baffling in the mixing vessel. As implied above, it is pre ferred to have the temperature of the molten metal comparatively cool so that the foaming gas is not expelled from the foaming agent at such a rate as to prohibit good foaming.

A modified metal hydride may be used to cut down the rate of hydrogen evolution. Zirconium hydride, hafnium hydride, and titanium hydride have the formulae ZrH HfH and TiH respectively. The articles of commerce generally contain somewhat less hydrogen than stoichiometric and may have 1.98 moles of hydrogen per mole of metal. We have found that better results are achieved if the hydrides are heat-treated prior to use. The heat treatment may comprise subjecting the hydride to temperatures of about 100 to about 400 C. for from about 1 to about 24 hours, preferably under an inert atmosphere (a blanket of nitrogen, argon, or a similar inert gas). It is more convenient to use ambient pressure than subatmospheric or superatmospheric pressures. These can be used if desired, however, and in many instances a change in the duration of the heating period and/or the temperature of the process is made. For example, using superatmospheric pressures, say up to about 1500 p.s.i.g., it is usually desirable to raise the process temperature and/or the length of heating time. Conversely, when using subatmospheric pressures comparatively lower temperatures and/or shorter heating periods are used. When the composition becomes from 1.65 to 1.80 moles of hydrogen per mole of metal, the product is a fine foaming agent which will yield hydrogen at a reduced rate. There is therefore more time admix the materials with the molten metal. This lead time therefore lends itself to more uniform mixing. Such mixing and the slower release of hydrogen gives smaller and more uniform pore size.

For the production of very superior foams using the process of this invention, the following five points are given as guidelines.

First of all, it is prefered to thicken at a temperature within a certain range above the liquidus temperature as stated above. The treatment temperature should be fairly constant throughout the mass of metal to be treated. In other words, for best results, there should not be a variation of temperature greater than say 150 C. throughout the mass of the molten metal to be thickened. 'In view of this, it is preferred to have the molten metal come to more or less an equilibrium before addition of the viscosity-increasing agent.

For best results, attention should be paid to good mixing technique. In other words, the geometry of the mixing vessel is not only determined by capacity requirements, but optimally, it is selected to work well with the mixing equipment used.

In order to conserve viscosity-increasing agent, good mixing is employed. Preferably the mixing technique should be one which moves or agitates substantially the entire mass of molten metal being treated, and efiicient stirring techniques are suitable in this respect. It is preferred to use stirring equipment which is efiicient at high viscosities, say 10,000 to 30,000 centipoises, and a Cowles Dissolver or Premier Hi Vis Dispersator with its high viscosity mixing head have been found to be adequate.

The addition of foaming agent is usually carried out at a temperature lower than addition of the viscosityincreasing agent. Thus, in most instances, it is preferred to cool the viscous melt before adding the foaming agent. In many instances, the cooling is best carried out in a second vessel, i.e., a vessel other than the hot chamber in which viscosity was increased.

The walls of the second vessel are preferably preheated to within C., preferably, :20" C. of the foaming temperature to be used. Thereafter, the viscous alloy is added thereto and allowed to come to temperature.

The foaming temperature is somewhat dependent upon the viscosity of the molten mass. For those masses which are highly viscous, it is preferred to use comparatively high temperatures. For less viscous materials, lower temperatures are employed. Since the presence of magnesium enhances viscosity, the foaming temperature will depend somewhat on magnesium concentration. When using a 1.2 weight percent magnesium alloy, temperatures about 650 C. are optimum. For aluminum based alloys having 2.2 and 7 weight percent magnesium, optimum temperatures for addition of foaming agent are about 635 C. and 615 C. respectively.

Some aluminum-silicon alloys are characterized by having greater fiuidities than pure aluminum. For these alloys temperatures of about 530 to 570 C. may be used when adding the foaming agent. Of course, as with other alloys, Al-Mg, for example, higher temperatures can be used if fluidity is compensated for by addition of more viscosityincreasing agent, such as CO When dispersing the foaming agent into the mass to be foamed, it is best to strive for quick mixing, which will afford more homogeneous foams. Hence, high speed mixing techniques which quickly disperse the blowing agent throughout the molten mass are preferred. These techniques are discussed above with regard to addition of the viscosity-increasing agent.

After the addition of the foaming agent, the mass is allowed to foam. This can be done in a variety of ways, for example, in an open or closed mold. If foaming occurs in an open mold, the exposed surface of the foamed mass may become irregular in shape. When effected in a closed mold, the resultant foam can achieve the shape defined by the mold.

Of course, for most purposes, the size of the foaming chamber should be sufiicient to contain the expanding mass. The temperature of the sides of the vessel (in which foaming takes place) should not be too cold. If they are, an undesirably thick unblown skin will be formed. If the sides of the vessel are too hot, buckling of the slab, or undesirably large holes will appear in the foamed interior. If too cold, incomplete foaming may occur. In short, by proper regulation of the mold temperatures the smoothness and thickness of the surface of the finished article can be regulated.

If substantial conformance of the foamed product to the size and shape of the reaction vessel is critical, then means should be taken to uniformly distribute the foaming mass into the mold. Thus, for example, in those instances where the admixing of foaming agent and metal did not take place in the mold in which foaming occurs, means should be taken to add the mass in a way to best fill the mold. For example, the mold can be agitated to get the mass into the mold corners.

The mold should not be chilled too suddenly, otherwise the foam will not be uniform and incomplete filling of mold may occur.

The foams produced by the process of the invention are useful as structural materials, for example, curtain walls.

When using substantially pure aluminum, air, carbon dioxide, or oxygen are the preferred thickening agents. Preferably for thickening one uses an aluminum base alloy. Alloys of choice contain one or more of the alloying elements mentioned hereinabove. The most preferred alloying element is magnesium. However, when preparing metal foams from the thickened mass, another preferred alloying element is titanium. When using metal alloys of the types hereinbefore described, one can use oxygen, nitrogen, air, argon, carbon dioxide, or water as the thickening agent. Tin promotes the thickening characteristics of copper-, magnesium-, and zinc-containing alloys. When using air or oxygen, usually an increase in temperature occurs upon introduction of the thickening substance into the metal mass. When using argon it is preferred that thickening be carried out at a temperature of from about 740 C. to about 780 C.

FURTHER ASPECTS OF FOAMING In general, aluminum magnesium alloys are preferred when preparing foams according to this invention. The resultant foams have a good balance of properties which make very attractive materials for the uses mentioned thereof. When using such alloys, foams characterized by having small pores of surprisingly uniform size can be prepared. Moreover, such high quality foams can be readily prepared with a density of to 16 p.c.f. In general, aluminum-magnesium foams are of a slightly higher density than foams prepared from substantially pure aluminum by the same method; and moreover, the Al-Mg foam walls are heavier. With regard to the thickness of cell walls, the presence of silicon, or titanium, tend to make them thin; while copper and zinc (as magnesium) tend to make them heavier. Furthermore, copper and zinc tend to increase the density of the foams. However, copper does not increase the density as much as magnesium on an equal weight of metal basis. Using aluminum-silicon alloys and aluminum-titanium alloys, foams having a density of 8-10 p.c.f. can be prepared. Thus, silicon is an advantageous alloying element even though aluminum-silicon melts are thinner than aluminum and this makes thickening and foaming somewhat more ditfcult.

Zirconium hydride is a preferred blowing agent for this invention, and it can be used very effectively with a wide variety of aluminum base metals. However, with some alloys containing silicon, titanium hydride gives more satisfactory results.

When foaming substantially pure aluminum, air, carbon dioxide, or oxygen generally give better results than water, argon or nitrogen. With substantially pure aluminum, a given amount of air or oxygen usually affords less thickening than that obtained with certain aluminum alloys, and in such instances, one may wish to use an increased amount of air or oxygen. At least with some samples of aluminum it has been noted that carbon dioxide is an exception to the general rule stated above, viz, that thickening is proportional to the amount of viscosity-increasing agent employed. More specifically, it has been found in these samples that the thickening due to carbon dioxide appears to pass through a maximum at around 2-4 grams of carbon dioxide per 100 grams of metal. When greater amounts are introduced, the thickening is first lessened somewhat but the thickness of the melt does not become reduced to the viscosity of the untreated aluminum. The viscosity can rise upon further CO addition; i.e., to levels greater than those causing diminishment. It has been noted that various metals promote the thickening characteristics of aluminum when alloyed therewith. Such metals are exemplified by magnesium, copper, and tin, and also zinc. Since alloys containing these metals thicken to a greater extent than substantially pure aluminum, it is clear the alloying elements promote the thickening process. The abilities of these promoter metals to promote thickening is also indicated by the ability to readily thicken alloys containing them with, for example, argon, water, or nitrogen. The discovery of the promoter aspects of these metals is an important feature of this invention. The mechanism of promotion is unknown.

As mentioned above, treatment with the viscosity-increasing agents of this invention can result in a molten mass so thick that it is resistant to foaming, and the viscosity increase tends to lessen over time. These properties can be taken advantage of when preparing a metal foam. Thus, it is not necessary to thicken to the extent desired to prepare foam and then conduct the foaming operation before the viscosity increase has lessened to an undesirable extent. Rather, one has the option of thickening to an extent greater than that desired, allowing the thickening to decrease to the optimum amount by allowing the molten mass to stand, and then conducting the foaming operation. In many instances, the last procedure is desirable inasmuch as it affords a means to control the amount of viscosity increase and to a great extent also increases the possible time interval between thickening and foaming. One can also thin a mass by adding silicon or titanium thereto.

As inferred above, the molten mass can be made too thick for optimum foaming. In other words, in many instances, the quality of the resultant foam is not directly proportional to the amount of thickening agent added over the entire possible concentration range of thickening agent. One reason for this is the inefficient stirring which will result if the molten mass is too thick. In other words, best results are achieved if the foaming agent is quickly and uniformly dispersed throughout the mass to be foamed. This type of dispersion i difiicult to achieve if the mass to be foamed is so thick that efiicient stirring cannot be carried out. Moreover, mass transfer or ability to fill a mold may be undesirably diminished.

EXAMPLE I A sample of a magnesium-aluminum alloy having 7 weight percent of magnesium and 0.2 weight percent of Mn weighing 3173 g., was melted. Nitrogen gas, at a flow rate of 8 litres per minute was bubbled through the molten alloy for five minutes. The nitrogen was admitted into the molten alloy through a ceramic tube about two inches below the surface. The alloy was stirred at about 2500 r.p.n1. during the nitrogen introduction. Stirring was commenced when the alloy was at 670 C., and at the end of nitrogen introduction the temperature was 550 C.

The alloy was heated to 725 C. and transferred to a holding furnace. The increase in viscosity noted at the end of the five minute introduction period was still apparent upon reaching 725 C. The alloy (prior to nitrogen introduction) had a true viscosity of about 13.8 cp. and, upon reaching 725 C., the viscosity was about 29 cp. However, the alloy was very resistant to flow.

The above procedure was repeated using a second batch of the alloy weighing 3185 g. The nitrogen flow rate was 7 litres per minute and the nitrogen introduction time was 5.4 minutes.

The two batches were combined in a pot heated to 670 C. The metal mass was allowed to cool to 680 C. The mass was stirred at 6000 to 10,000 rpm. and 40 grams of zirconium hydride, ZrH was admixed over an introduction period of 8.6 seconds. Thereafter it was cast into a mold. The mold capacity was about 8 to 9 times as big as the volume of the unblown liquid combined batches.

The mixture foamed to fill the (closed) mold. The resultant foam was sectioned demonstrating a fine pore, quite uniform structure having a density of about 25 pounds per cubic foot (a density of about 15 percent of unfoamed alloy).

Similar results are also obtained when the following variation are made:

The amount of nitrogen used is from 40 to 60 liters per 3000 grams of alloy.

The nitrogen is introduced over a period of from 2 to 5 minutes.

The nitrogen is introduced into the alloy at temperatures of from 650 to 710 C.

The alloy has from 1.2 to 35 weight percent of magnesium, silicon, manganese, copper, titanium, or tin.

From 0.2 to 1.0 gram of TiH ZrH or HfH are used per each gram portion of alloy.

When the nitrogen gas was introduced into the molten alloy, a white smoke was noted above the alloy. In addition, the stirrer shaft exposed to this atmosphere became coated with a fine white powder analyzing as magnesium oxide. This formation of magnesium oxide is not noted when carbon dioxide or air is used as the viscosity-increasing agent. Evidently the nitrogen sweep puts some magnesium into the atmosphere which reacts with the oxygen there to form the white smoke.

EXAMPLE II This example was conducted on three batches of an aluminum-magnesium alloy having 7 weight percent of magnesium and 0.2 weight percent of Batch (a) of said alloy, 3130 g., was heated to 715 C. Using a hollow stirrer shaft and good mixing, argon was introduced into the molten alloy. The rate of argon introduction was 16 liters/minute; the total time of argon fiow was 3 minutes. The argon introduction cooled the alloy to 604 C. The alloy was very resistant to flow. This gross lowering of fluidity was also apparent after heating the alloy to 704 C.

Batch (b) of said alloy, 1430 g., was melted and heated to 704 C. Through the hollow stirring shaft, 16 liters/ minute of argon were introduced for two minutes. Such introduction of argon cooled the alloy to 649 C. The high viscosity noted was still present upon reheating the alloy to 738 C.

Batch (c) of said alloy, 1730 g., was heated to 715 C. and while stirring effectively, 16 liters of argon/minute were introduced through the hollow stirring shaft for two minutes. This cooled the melt to 682 C., at which temperature the melt was very viscous.

All three batches of viscous alloy were combined in a pot held at 730 C. The metal was allowed to reach 685 C. At that temperature, 40 grams of ZrH were added and rapidly dispersed for 6.7 seconds.

The thickened mass was transferred by gravity to a x 15 x 4 /2 inch mold. (The mold temperature was about 300 C.) The thickened mass was too thick to flow well by gravity into the mold; thus, only about 85 percent of the mass reached the mold.

The mold was closed with a lid. It was held for five minutes at 300 C., then removed from the mold chamber. The foamed casting was removed from the mold.

Sectioning revealed the casting was a substantially uniform, fine pore foam having a density of 18 to pounds per cubic foot.

Similar results are obtained when the following variations are made:

From 1 to 4 grams of argon is used per each 100 grams of alloy.

From 0.6 to 1.2 grams of Zrl-I is used per 100 grams of alloy.

The argon is introduced over a 1 to 4 minute period.

The temperature of the alloy is from 660 to 710 C.

EXAMPLE III A 2265 gram sample of the alloy used in Example II was heated to 1300 F. While excluding air, and while stirring, 11 liters of nitrogen/minute were introduced into the molten mass for five minutes. (The nitrogen was admitted through a hollow stirring shaft while using a stirring rate of 2000 to 3000 rpm.) The melt was still very viscous upon reheating to 1380 F.

EXAMPLE IV A sample of 2120 g. of the alloy used in Example III was heated to 1300 F. While stirring with a stirrer having a hollow stirring shaft at a rate of 2000 to 3000 rpm. argon gas was introduced into the molten mass through the shaft at a rate of 16 l./minute. The argon introduction was carried out at this rate for five minutes.

The gas flow reduced the temperature of the mass to 1300 F. and markedly increased the viscosity of the molten metal. High viscosity increase was still apparent when the molten alloy was reheated to 1400 F.

1 2 EXAMPLE v A 3235 g. sample of an aluminum alloy having 7 weight percent of Mg and 0.2 weight percent of Mn was heated to 1300 F. and 28 grams of CO gas were admitted to the molten melt over 2.5 minutes. The CO addition was carried out while stirring vigorously and the CO was introduced through a copper dip tube. The lower two inches of dip tube eroded during the addition. The temperature of the molten alloy was reduced to 1180 F. during the gas addition, and its viscosity markedly increased.

Another sample of the alloy, 3225 grams, was heated to 1300 F. and treated with gaseous CO This time, about 3 inches of dip tube dissolved in the melt. The CO added was grams and the addition time was 2.7 minutes. Viscosity was markedly high and the temperature of the melt dropped to 1100 F. during the CO addition.

The two batches of viscous melt were combined in a mixing pot which was at a temperature of 670 C. When the metal mass reached a temperature of 685 C., 40 grams of ZrH- were added while efiiciently stirring at 6000 to 10,000 rpm.

(During the addition of the ZrH a mechanical mishap occurred whereby the stirring shaft fell into the mix pot.) Nevertheless, the hydride-dispersed metal alloy was then fed by gravity to a 15 x 15 x 4 /2 mold which was at 575 F. Because of the high viscosity, the gravity feed to the mold was poor and a large part of the melt was held up. Thus, only about 85 percent of the mold was filled.

A very fine-pored uniform foamed casting having a density of 25 p.c.f. was obtained after cooling and opening the mold.

The above example is repeated in one batch of 6460 grams of aluminum alloy and only 3.23 grams of CO A good foam is obtained. The example is repeated again in a single batch, this time using 258 grams of CO yielding a good foamed product.

Example V is again repeated in a single batch of 6460 grams of aluminum alloy, this time using 323 grams of oxygen as the thickening agent. A good foamed aluminum is obtained. It is repeated again, this time using only 1.29 grams of oxygen as the thickening agent.

Example V is repeated in a single batch of 6460 grams of aluminum alloy using 646 grams of air as the thickening agent. A useful foamed aluminum is obtained. Likewise, when only 12.9 grams of air is used as the thickening agent a good foam is obtained.

Similar results are also obtained when the alloy to be formed contains 2-10 percent magnesium, 0.81.2 percent titanium, 8-12 percent copper, 8-12 percent titanium, 8-12 percent copper, 8-12 percent zinc, 0.4-O.8 percent manganese, 1-2 percent tin, or 0.4-2 percent silicon. Similar results are also obtained when such alloys are thickened with from 1 to 8 grams of C0 1 to 6 grams of water, 0.5 to 3 grams of air, 1 to 4 grams of nitrogen of nitrogen, 1 to 4 grams of argon or 1 to 5 grams of oxygen per each 100 gram portion of metal and the thickening is conducted at a temperature 20 to C. above the alloys liquidus temperature. Comparable results are obtained when TiH or HfH are used as the foaming agent or when the foaming agent is a non-stoichiometric hydried TIHL65, HfH1 n, TiH go, ZI'HL65, or ZI'HL3O, the blowing agent is used in amounts of from 0.2 to 1.0 gram per each gram portion of metal, and the foaming is conducted from 650 to about 705 C.

EXAMPLE VI A 3355 g. sample of an alumnum magnesium alloy containing 7 weight percent of Mg and 0.2 weight percent of Mn was heated to 1320 F. While efficiently stirring, air was introduced into the alloy for 3.5 minutes at a rate of 16 l./minute. This markedly increased the viscosity of the molten metal while decreasing the temperature to 1160 F. Marked viscosity increase was observable when the molten alloy was heated to 1340 F.

Another 1260 g. batch of the same alloy was heated to 1260 F. While stirring efficiently, air was added to the alloy for minutes at a rate of 15.5 liters per minute. This decreased the temperature to 1060 F. and markedly increased the viscosity.

Using a mixing pot at 695 C., the combined batches of viscous alloy were brought to 679 C. and foamed with grams of ZrH The hydride was admixed for 5.1 seconds with efficient stirring.

The hydride-treated melt was put into a mould as in the previous examples. A fine-pored, fairly uniform foamed casting was obtained.

Similar results are obtained when the following variations are made:

The amount of air is from 0.5 to 3 grams per 100 grams of alloy.

The alloy contains from 1.2 to weight percent of magnesium, silicon, or tin.

The air is admixed at a temperature of from 20 to 90 C. above the liquidus temperature of the alloy.

The amount of Zirconium hydride is from 0.6 to 1.2 grams per 100 grams of alloy and the hydride is added at 670 to 705 C.

EXAMPLE VII A 1500 gram sample of an aluminum alloy having 7.5 weight percent of magnesium and 0.2 weight percent of manganese was heated to 690 C. While efficiently stirring, 90 ml. of H 0 was added to the molten metal over a five-minute period. The water was added while the temperature of the metal was 1250 to 1300 F.

The water, increased the viscosity of the melt. The melt was transferred to a pot at 610 C. and just before foaming the pot temperature was 690 C. Ten grams of zirconium hydride were quickly added with efiicient stirring at 4500 to 6500 r.p.m.

The hydride treated melt was allowed to foam in the mixing pot to yield a fine pore foam.

Similar results are obtained when from 1 to 6 grams of H 0 and 0.6 to 1.2 grams of zirconium hydride is used per 100 grams of alloy containing 1.2 to 35 weight percent of silicon, tin, or magnesium, when the H 0 is added at a temperature of 20 to 90 C. above the liquidus temperature of the alloy and the ZrH is added at 670 C. to 705 C.

EXAMPLE VIII Using a 1627 gram portion of an alloy containing 7 weight percent of magnesium and 0.2 weight percent of manganese, the following experiment was conducted:

The alloy sample was heated to 740 C. and made viscous with CO (solid). The thick material was transferred to a mixing pot at 690 C. and while vigorously stirring, 20 grams of HfH were added. Some minor detonations occurred. The sample was allowed to foam in the mixing pot.

There is another utility conferred by the CO treatment. Specifically, such treatment increases the luster of the alloy. In other words, alloys which have been treated with CO are brighter to the eye than the same alloy not so treated. The increased brightness is apparent in alloy masses which have been cooled to the solid state. The brightness persists after other processing, such as foaming the molten alloy.

To illustrate this, two aluminum-magnesium alloy foams were prepared. Foam B was made from alloy treated with CO while Foam A was prepared from molten alloy not so treated. The reflectivity of each foam was determined spectrophotometrically. In the test, the reflectance was relative to magnesium carbonate. The results reported below in Columns A and B are percent diffuse reflectance relative to magnesium carbonate (MgCO =100 percent diffuse reflectance) at the wavelengths shown.

As shown by the data, Foam B (which was made up of CO -treated alloy) reflected more light throughout a wide spectral range.

The diffuse reflectance data was obtained using a Beckman Model B spectrophotometer with Beckmans diffuse reflectance accessory.

The formation of increased luster is not dependent on foaming. Thus, luster increase is present when the alloy is molten or solid. Solid objects with increased luster are prepared by merely allowing the molten CO -treated alloy to cool. The alloy may be cast into desired shape to form a desired object having utility where greater luster is desired.

EXAMPLE IX Five hundred grams of an aluminum alloy having the following alloying metals Weight percent Magnesium 7.04 Manganese .01 Silicon .08

Copper .02 Titanium .14

was heated to 650-720 C. in an induction furnace. The molten alloy was stirred at from 600 r.p.m. The stirring was somewhat inefficient inasmuch as the whole mass of molten alloy was not being thoroughly agitated. While stirring, about 200 grams of solid carbon dioxide was added.

A marked increase in viscosity was noted at these temperatures-650720 C.

After solidification and cutting, there was no pore structure typical of a metal foam.

Similarly, viscosity increases were noted when the experiment was repeated using the following aluminum alloys.

(1) 5.5-7 percent Sn 0.7-1.3 percent Cu 0.7-1.3 percent Ni maximum impurities 0.7 percent Si, 0.7 percent Fe, 0.1 percent Mn, 0.2 percent Ti; total others 0.3 percent 6.5-7.5 percent Si 0.5 percent Fe 0.2 percent Cu 0.1 percent Mn 0.2-0.4 percent Mg 0.2 percent Zn 0.2 percent Ti 0.15 percent others.

Viscosity increases were also noted using solid CO and the following alloys having the indicated amounts of metals alloyed with aluminum. The stirring conditions were similar to those used above.

(A) 4.5 percent Cu, 1.5 percent Mg, 0.6 percent Mn (B) 4.0 percent Mg, 0.5 percent Mn (C) 5.5 percent Zn, 2.5 percent Mg, 1.5 percent Cu, 0.3

percent Cr.

The conditions employed were as follows:

Mixing Amount, grams of time, Temperature, degrees minutes 002 Metal 15 The above viscosity increases Were obtained utilizing comparatively large amounts of CO Much less CO can be used if more efiicient stirring is utilized. The following examples illustrate this.

EXAMPLE X Samples of aluminum-magnesium alloys having 1, 2, 3, 4 and 7 weight percent magnesium were prepared by alloying the requisite amount of magnesium to a virgin aluminum having the following composition:

Percent Aluminum 99.78

Silicon .05 6

Iron .14

Gallium .015

02 Amount addition CO 9,

Percent time, required, magnesium minutes grams "Did not thicken.

A sample of virgin aluminum of the above composition and with no magnesium added was thickened with carbon dioxide in the procedure discussed below. For this reason, it is clear that carbon dioxide can thicken substantially pure aluminum as well as aluminum having magnesium in a concentration of less than 2 percent. Thus, it appears that more CO would have foamed the sample with 1 percent magnesium within the above table.

The aluminum sample, 6345 g., was heated to 760 C. Then 1143 g. of CO snow was added in 3 approximately equal proportions. Each portion decreased the temperature about 100 C. and the temperature was raised by reheating to 760 C. before the second and third portion were added. Each portion was added while stirring at about 2500 r.p.m. The three CO addition times totaled minutes.

After the last addition the sample of aluminum was reheated to 785 C.; no loss of thickening was observed. (The amount of thickening was about the same as that in Example 12 below.) Thereafter, it was transferred to a mix pot which had been previously heated to 690 C. The melt was allowed to cool (from 745 C.) to 690 C. over a 4-minute period.

While stirring at 9000 rpm, 38 grams of zirconium hydride were added to the thickened melt. The zirconium hydride was added in 6 aluminum foiled-wrapped packets of approximately equal amounts, stirring was continued for 4.0 seconds; thereafter, the resultant mass was added to a mold having dimensions 15 x 15 x 4 /2". A lid was put on the mold and was allowed to cool to ambient temperature. A fine pore aluminum foam casting was produced. This preparation shows that comparatively high amounts of the thickening agents of this invention can be utilized to thicken materials somewhat resistant to viscosity increase. In such instances, up to 16, 18, or more grams of thickening agent such as carbon dioxide, and the other agents previously mentioned can be utilized.

Similarly, alloys of aluminum having up to percent magnesium and aluminum-tin alloys having 1.2-35 percent tin, as well as aluminum-silicon alloys having 12-35 percent silicon, can be thickened using CO addition times 16 of 1-3 minutes and 005-8 grams CO per grams alloy. The temperature of the molten alloy is 20-90 C. above the liquidus point when the CO is added.

EXAMPLE XI A 3235 g. portion of an aluminum alloy having the composition 7 weight percent magnesium, 0.2 weight percent manganese (balance aluminum and impurities) was heated in an induction furnace to 705 C. It was stirred using a settling of 55 volts on the aforementioned stirrer. Gaseous CO 28.3 grams, was introduced 45" beneath the surface of the molten mass over a 2 /2 minutes period using A1" copper tubing. A high increase in viscosity was produced. Similar results are obtained using 0.5 to 1.5 grams of CO per 100 g. of alloy.

This and the other examples dealing with use of CO demonstrated that the physical state of the CO employed is not critical. Using the procedure of the above example, liquid CO can be added when employing pressure and temperature conditions under which CO is a liquid. (The melting point of CO is 56.6 C. at 5.2 stm.)

In some instances it appears that gaseous CO is more efficient than solid CO In other words, in these instances, it appears that less gaseous CO is required (to yield a given viscosity increase) than solid CO One such instance is when the alloy employed in Example III above is used.

EXAMPLE XII A 6,365 gram portion of virgin aluminum was heated to 740 C. The aluminum had the following analysis.

Wt. percent Aluminum 99.78 Silicon 0.56 Iron 0.14 Gallium 0.015

While stirring at 2000 rpm, oxygen gas was bubbled into the molten aluminum over a period of 4 minutes and 36 seconds such that a total of 4.1 cubic feet of gas was introduced. At the end of the oxygen addition, the temperature of the molten mass had decreased to 720 C. and a marked increase in viscosity was observed.

The thickened melt was reheated to 785 C.; no loss of thickening was observed. Thereafter, it was transferred to a mix pot which had been previously heated to 690 C. The melt was allowed to cool (from 745 C.) to 690 C. over a 4-minute period.

While stirring at 9000 rpm, 38 grams of zirconium hydride was added to the thickened melt. The zirconium hydride was added in 6 aluminum foil-wrapped packets of approximately equal amounts, stirring was continued for 4.0 seconds. Thereafter, the resultant mass was added to a mold having dimensions 15 x 15 x 4 /2". A lid was put on the mold and was allowed to cool to ambient temperature. A fine pore aluminum foam casting was produced.

EXAMPLE XIII A 6390 gram portion of virgin aluminum having the same analysis employed in Example XI was heated to 740 C. Thereafter, 7.1 cubic feet of air was added through a hollow-shaft stirrer over a period of 4.5 minutes. While the addition was carried out, the mixture was stirred at a rate of 2500 r.p.m. After addition was complete, the temperature of the molten (and now thickened) mass had decreased to 710 C. The resultant mass was heated to 770 C. and no loss in viscosity was observed.

The mass was then transferred to a mix pot and allowed to cool to 690 C. Thereafter, 38.5 grams of zirconium hydride was added in six aluminum foil-wrapped packets of approximately equal weight of hydride. Stirring at about 9000 rpm. was continued for 5.4 seconds after the addition of the hydride.

Thereafter, the foaming mass was transferred to a mold 15 x 15 x 4 /2", and the lid was placed on the mold.

A set cellular aluminum foam having small pores had a density of approximately 18 p.c.f. was produced.

1 7 EXAMPLE XIV A 13,750 gram portion of Almag 35 was heated to 760 C. Thereafter, 376 grams of carbon dioxide snow was added with stirring at 2500 rpm. The addition of the carbon dioxide caused a decrease in temperature to about 700 C.

The mixture was then reheated to 780 C. and transferred to a mix pot and allowed to cool to 682 C. At that temperature 82 grams of zirconium hydride and aluminum foil-wrapped packets (of approximately equal amounts of hydride) was added. After addition, stirring was conducted at 1000 r.p.m. for 6 seconds and then for 3.9 seconds at 1220 r.p.m. After that period the foaming mass was transferred to a mold 26 X 26 X 3%" which mold was at 355 C. The mold was filled completely, and the lid was placed on the mold.

On cooling to ambient temperature a medium pore size set cellular foam was produced. Samples of this foam having the dimensions 2 /2 X 12 x in size were obtained by sawing the set cellular product. These samples had a density of 16.4, 17.1, 17.8, 19.0, and 21.8 p.c.f.; and a flexural load strength of 45, 52, 65, 78, and 90 pounds respectively. The load strength was determined by ASTM test D790-66.

What is claimed is:

1. Process for increasing the viscosity of molten substantially pure aluminum with a gaseous viscosity-increasing agent, said process comprising introducing into said aluminum at a temperature of from about to about 90 C. above the liquidus temperature of said metal from about 0.02 to about 10 grams of air per each 100 gram portion of metal.

2. Process for increasing the viscosity of molten substantially pure aluminum with a gaseous viscosity-increasing agent, said process comprising introducing into said aluminum at a temperature of from about 20 to about 90 C. above the liquidus temperature of said metal from about 0.02 to about 5 grams of oxygen per each 100 gram portion of metal.

3. Process for increasing the viscosity of molten substantially pure aluminum with a gaseous viscosity-increasing agent, said process comprising introducing into said aluminum at a temperature of from about 20 to about 90 C. above the liquidus temperature of said metal from about 0.05 to about 8 grams of carbon dioxide per each 100 gram portion of metal.

4. Process of claim 3 wherein said carbon dioxide is solid carbon dioxide.

5. Process for the preparation of a metallic article having increased luster, said process comprising (i) adding from about 0.05 to about 8 grams of carbon dioxide to each 100 gram portion of a molten aluminum-based alloy at a temperature of from about 20 to about 90 C. above the liquidus point of said alloy, said alloy containing from about 2 to about weight percent of a promoter selected from the class consisting of magnesium, silicon, and tin; and (ii) and, without foaming, subsequently cooling the resultant viscous mass to solidify said mass.

References Cited UNITED STATES PATENTS 3,214,265 10/1965 Fiedler -20 F 3,300,296 1/1967 Hardy et a1. -75-20 F 3,379,517 4/1968 Graper 75-20 F 3,705,030 12/1972 Berry et a]. 75-20 F WALTER R. SATTERFIELD, Primary Examiner US. Cl. X.R.. 75-68 

1. PROCES FOR INCREASING THE VISCOSITY OF MOLTEN SUBSTANTIALLY PURE ALUMINUM WITH A GASEOUS VISCOSITY-INCREASING AGENT, SAID PROCESS COMPRISING INTRODUCING INTO SAID ALUMINUM AT A TEMPERATURE OF FROM ABOUT 20* TO ABOUT 90* C. ABOVE THE LIQUIDUS TEMPERATURE OF SAID METAL FROM ABOUT 0.02 TO ABOUT 10 GRAMS OF AIR PER EACH 100 GRAM PORTION OF METAL. 